The present invention relates to a photoelectric transfer material, its manufacturing method, photoelectric transfer element and its manufacturing method, especially suitable for application to organic solar cells, for example.
Existing solar cells using organic dyes are roughly classified by two types. Those of one type are dye-sensitized solar cells having a structure in which a dye is made to adhere on an oxide semiconductor on an electrode and an electrolyte is injected between the electrode and a counter electrode. Those of the other type are organic solar cells having a structure in which electron-acceptable compound of an organic dye such as phthalocyanine and pellerine or the like is deposited in form of a film on a light permeable electrode by vapor deposition or spin coating (Appl. Phys. Lett. 1986, 48, 183).
The former-type dye-sensitized solar cells have been subjected to vigorous studies by Gratzel et al. and have attained the efficiency around 10% (AM 1.5) (Chem. Commun. 1997, 105). However, because of the difficulty in sealing the electrolytic solution, it is difficult to employ this cell structure to flexible solar cells using a plastic substrate. Therefore, researches move forward to solidify electrolytes.
The latter-type organic solar cells could not exceed 1% in efficiency for a long time because of high internal resistance inherent to organic compounds and small thickness of the layer contributing to separation of electric charge. In 1995, however, Heeger et al. reported an organic cell made of a conductive polymer and a fullerene derivative. After the organic cell was proved to achieve much higher performance than conventional organic solar cells, researches of this line have become active. In conventional organic solar cells, cells were formed by depositing electron acceptors and dye molecules in form of layers. However, since separation of electrons occurs at the interface of these layers as explained above, only some of photons captured near the interface could be effective for separation of electrons. Moreover, since the charge generated thereby had to move in the high-resistance organic substance to respective electrodes, internal resistance of the cell increased and made it difficult to extract a large current. In the above-introduced organic solar cell composed of the conductive polymer and the fullerene derivative, the polymer and the fullerene derivative merge in the molecular level, and result in successfully forming a very large interface. With organic solar cells of this type, efficiency around 2.5% (AM 1.5) has been reported heretofore in the system of MDMO-PPV:PDBM (Appl. Phys. Lett. 78(6), 841-843; U.S. Pat. No. 5,331,183; and Science, 1995, 1789).
All of solar cells of this type are composed of flexible organic compounds and have various advantages, namely, excellent flexibility, no requirement of annealing process such as calcination, readiness for being made by coating process such as spin coat method except the back electrode (such aluminum), dependency of the potential simply upon the potential difference between the conductive polymer as the electron donor and the electron acceptor, possibility of making a large interface because of the bulk heterojunction.
Japanese Patent Laid-open Publication No. JP2002-335004A describes introducing a kind of porphyrin as a photosensitizer into a photoelectric transfer material containing a conductive polymer as the electron donor, spherical shell-shaped carbon as the electron acceptor, linear or tubular carbon as the electron transporter. This publication, however, does not describe introduction of antenna porphyrin aggregates as a kind of porphyrin.
A method of forming annular associated porphyrin is disclosed in J. Am. Chem. Soc. 2003, 125(9), 2372-2373. Methods of cross-linking porphyrin units are disclosed in Japanese Patent Laid-open Publications No. JP2002-281616A and No. 2003-54719A. Characteristics of porphyrin aggregates are described in Angew. Che. Int. Ed. 2000, 39(22), 4070-4073 and Chem. Commun. 2002, 1104-1105.
However, the above-mentioned systems have disadvantages such as still large electric resistance of films (trade-off of thickness of films and light absorbability), inefficient use of sunlight because of mismatch of absorption spectrum of MDMO-PPV with sunlight spectrum, instability in air because of liableness to oxidation of MDMO-PPV and their still high costs.